PROJECT 2 PROJECT SUMMARY Ovarian high-grade serous cancer (HGSC) is the most lethal gynecological malignancy. More than 80% of HGSC patients recur after standard chemotherapy. We have identified a novel and highly active genotoxic therapy by co-inhibiting poly-ADP ribose polymerase (PARP) and ATR checkpoint kinase. Combination PARP inhibition with ATR inhibition (PARPi-ATRi) synergizes to specifically target and kill ovarian HGSCs harboring common HGSC-associated alterations, e.g. homologous recombination (HR) deficiency and Cyclin E overexpression. Our preliminary studies show that PARPi-ATRi in combination is especially effective in killing tumor cells with these alterations and even causing regression of HR-deficient and Cyclin E overexpressing ovarian HGSCs. In the clinic, PARP inhibition (PARPi) treatment alone for ovarian cancer alone results in partial tumor regression and rarely complete responses with the ultimate emergence of drug resistance. This proposal addresses this urgent clinical need by using a potent new combination treatment to convert partial responses with PARPi monotherapy into complete and durable tumor regression. For these studies, we have developed: 1) >60 PDX models representing the clinically most common and challenging conditions to treat including: HR-deficient, PARPi-resistant and Cyclin E overexpressing ovarian HGSCs with differing platinum sensitivities, 2) a novel PARPi tracer that will be tested as a predictive and pharmacodynamic marker to guide patient selection for PARPi therapies, 3) advanced proteomic methods to detect both global-tumor and replication fork-specific responses to treatment. We hypothesize that dual inhibition of PARP and ATR will increase the frequency of complete tumor regression in ovarian cancer compared to PARPi monotherapy. The proposed studies herein will test the efficacy of PAPR inhibitor (PARPi, olaparib), by combination with ATR inhibitor (ATRi, AZD-6738) in the first clinical trial in ovarian cancer supported by our preclinical data. Secondly, we will identify dosing schedule strategies to minimize drug toxicity without compromising efficacy for PARPi-ATRi in PDX models. Combination PARPi-ATRi has shown efficacy and tolerability in early phase IB trials, but ways to decrease toxicity are important to optimize quality of life for these patients. Finally, we will perform genomic and proteomic studies to identify biomarkers of PARPi-ATRi response for evaluation in future clinical trials. Our Hopkins?PENN SPORE team is comprised of: expert clinical trialists, translational scientists with preclinical models and drug optimization expertise, molecular biologists with expertise in DNA replication stress, SPORE Cores such as Pathology that will promote optimal patient tissue procurement and processing and Biostats to oversee data analysis. Thus, our team is well positioned for success with realizing Project 2 goals.